1. Field of the Invention
The present invention generally relates to a belt driving control device configured to control a belt looped over plural rollers, a belt device having the belt driving control device, a belt driving control method, and an image forming apparatus having the belt device.
2. Description of the Related Art
In the related art, there is provided an image forming apparatus having such a belt, such as a photoreceptor belt, an intermediate transfer belt, and a sheet transfer conveyor belt. In this type of image forming apparatus, high degrees of accuracy in controlling the drive of the belt may be a prerequisite in order to insure high image quality. Specifically, in a tandem type image forming apparatus having a direct transfer system capable of exhibiting an excellent image forming speed and suitable for reduction in size, high degrees of accuracy may be required for controlling the drive of a conveyance belt that conveys a recording sheet (i.e., a sheet-type recording medium). In the image forming apparatus, the recording sheet is conveyed by the conveyance belt and sequentially passed through each of plural image forming units arranged along a conveyance direction such that mutually different homochromatic images are formed on the recording sheet. The different homochromatic images are individually superimposed on one over another to thus form the color images on the recording sheet.
FIG. 8 shows one example of the tandem type electrophotographic image forming apparatus having an intermediate transfer system. Reference numerals 22, 23, 24 respectively indicate a conveyance belt, a driving roller, and a driven roller. Further, reference numerals 21Y, 21M, 21C, 21K each indicate one of image forming units.
In the image forming apparatus, for example, the image forming units 21Y, 21M, 21C, and 21K that form corresponding homochromatic images in colors of yellow (Y), magenta (M), cyan (C), and black (K) are arranged in a travelling direction of conveyance of a recording sheet. Electrostatic latent images formed on surfaces of unshown photoconductor drums 12Y, 12M, 12C, and 12K are then developed by exposure of laser from a laser exposure unit 21 at the corresponding image forming units 21Y, 21M, 21C, and 21K to form corresponding homochromatic toner images (perceivable images). The homochromatic toner images are attached to the conveyance belt 22 by electrostatic force, and sequentially transferred from the conveyance belt 22 onto the recording sheet (not shown) such that the homochromatic toner images are sequentially superimposed on the recording sheet. Thereafter, toner of the superimposed images are fused and pressed by a fixation device 17, thereby forming color images fixated on the recording sheet.
The conveyance belt 22 is looped with adequate tension over a driving roller 23 and a driven roller 24 arranged in parallel with each other. The driving roller 23 is rotationally driven at a predetermined rotational velocity by unshown driving motor and the conveyance belt 22 endlessly travels at a predetermined velocity according to the rotation of the driving roller 23. The recording sheet is supplied by a paper-feeding mechanism on a portion of the conveyance belt 22 located at a side where the image forming units 21Y, 21M, 21C, and 21K are arranged, and conveyed at the same velocity as the travelling velocity of the conveyance belt 22. Thus, the recording sheet is passed through each of the image forming units arranged in series.
In the image forming apparatus, failure to maintain the travelling velocity of the recording sheet or the travelling velocity of the conveyance belt 22 at a constant value results in color shifts. Such color shifts result from relative shifts in transferring positions of the homochromatic images that are alternately superimposed on the recording sheet. The color shifts may result in blurring fine line images formed by superimposing images of plural colors or white dot defects around profiles of black character images in the background image formed by superimposing images of plural colors.
FIG. 9 shows another example of the tandem type electrophotographic image forming apparatus having the intermediate transfer system. In the intermediate transfer system of the tandem type electrophotographic image forming apparatus, the homochromatic images individually formed on the corresponding surfaces of photoconductor drums 12Y, 12M, 12C, and 12K of the image forming units 21Y, 21M, 21C, and 21K are temporarily transferred on the intermediate transfer belt 16 such that the images are sequentially superimposed on one over another, and the superimposed images are transferred on the recording sheet at once.
In this tandem type electrophotographic image forming apparatus having the intermediate transfer system, the color shifts may also be generated if the intermediate transfer belt 16 does not travel at a constant velocity. In the image forming apparatus, including the aforementioned tandem type image forming apparatus, utilizing a belt as a conveyance member for conveying a recording material or as an image carrier including a photoconductor or an intermediate transfer member for carrying the images transferred on the recording material, failure to maintain the travelling velocity of the belt at a constant value may result in banding. The banding indicates image density heterogeneity that results from fluctuations in the travelling velocity of the belt while images are being transferred on the recording material. Specifically, a portion of the image transferred on the intermediate transfer belt 16 when the travelling velocity of the belt is relatively fast has a profile extended in a circumferential direction (i.e., travelling direction) of the belt whereas a portion of the image transferred on the intermediate transfer belt 16 when the travelling velocity of the belt is relatively slow has a profile shrunk in the circumferential direction of the belt, in comparison to the original profile of the image. The extended portion of the image has low density while the shrunk portion has high density. As a result, the image density heterogeneity in the circumferential direction of the belt or banding is observed. The banding is clearly perceived with the naked eye when pale homochromatic images are formed.
The travelling velocity of the belt varies with various factors including inconsistency of a belt thickness if the belt is formed of a single layer. The thickness inconsistency of the belt results from thickness variation of the belt in the circumferential direction which is manufactured with a cylindrical mold by a centrifugal baking system. In the belt having such a thickness variation in the circumferential direction of the belt, the travelling velocity of the belt increases when a thicker portion of the belt is looped over the driving roller whereas the velocity decreases when a thinner portion of the belt is looped over the driving roller. The thickness inconsistency or thickness variation results in fluctuation in the travelling velocity of the belt.
Japanese Patent Application Laid-Open No. 2006-264976 discloses an image forming apparatus having a belt driving control device capable of controlling such thickness inconsistency of the belt. In the disclosed image forming apparatus, a correcting amount for the driving roller to control is computed based on information on the rotational angular displacements or rotational angular velocities of the two rollers individually having different diameters. The rotational velocity of the driving roller is then controlled based on the computed correcting amount obtained in order to cancel out the fluctuation in the belt travelling velocity due to the thickness variation of the belt in the circumferential direction of the belt. The disclosed document also includes a method of synchronizing the correcting amount with the thickness fluctuation, in which a reference mark is provided on the belt as a home position, and the marked home position on the belt is scanned by an optical sensor or the like to detect the home position. In the disclosed document, there is a method of synchronizing the correcting amount with the thickness fluctuation without having the reference mark placed on the belt as the home position. In this method, the accumulated value of the rotational angular velocity of a roller obtained by an encoder mounted on the roller supporting the belt is computed. A virtual home position is then determined as a position at which the computed accumulated value has reached a prescribed value.
However, the belt driving control device in the image forming apparatus according to the related art cancels out the fluctuation in the belt travelling velocity resulting from the thickness variation of the belt only after the home position or the virtual home position of the belt has been detected. In this case, though depending on where the home position of the belt is, the belt control device may have to wait (i.e., waiting time) until the belt has been driven to travel one rotation to reach the home position again in order to cancel out the fluctuation in the belt travelling velocity. In the image forming apparatus such as a printer or a copier having the belt driving control device, it is desirable that duration of time (hereinafter called “first print time”), which indicates the time from the time at which the print starting command given by an operation unit or a superordinate apparatus is received to the time at which the printed matter is discharged onto a discharge tray of the image forming apparatus, is as short as possible. In order to provide uniform image quality, it is generally preferable that images be formed after fluctuation of the belt travelling velocity has been stabilized. Accordingly, in such an image forming apparatus, it is desirable that the aforementioned waiting time is made as short as possible because the waiting time is directly added to the first print time.